In fabricating filters for use in connection with cigarettes and the like, a number of different properties of the resultant filter must be taken into consideration. While filtration efficiency (i.e., the ability of the filter to remove undesirable constituents from tobacco smoke) is perhaps the most important property of cigarette filters, filtration efficiency must frequently be compromised in order for the filter to possess a commercially acceptable combination of other properties, including pressure drop, taste, hardness, appearance, and cost. For example, the most commonly utilized cellulose acetate filter has a relatively low filtration efficiency since increased efficiency can only be obtained either by increasing the density of the filter material or the length of the element, both of which produce a pressure drop across the filter which is excessive and unacceptable from a commercial standpoint.
In recent years, air dilution has become a popular technique for compensating for the relatively low filtration efficiency of cigarette filters having a sufficiently low pressure drop for commercial acceptance. The air diluation technique employs air to dilute the smoke stream from the cigarette and thereby reduce the quantity of tar and other undesirable tobacco smoke constituents drawn into the smoker's mouth for each puff or draw. The air is generally provided through a plurality of perforations at the tipping paper employed for joining the filter to the tobacco column of the cigarette, and if the filter is overwrapped with plug wrap paper, an air pervious plug wrap paper is employed. Alternatively, air-impervious plug wrap paper may be employed and grooves formed in the air impervious plug wrap paper to provide passages of air to the smoker's mouth.
The demand for low tar cigarettes has become a major portion of the cigarette business and, therefore, the air dilution technique has become a primarily important approach in achieving low tar levels. As noted above, however, all prior art air dilution filters employ either perforated air-impervious tipping paper or porous tipping paper to permit air to flow through the tipping paper and either into the main smoke stream or along a groove defined in the rod periphery. In these prior art air dilution filters, the ultimate control of tar delivery is dependent upon the uniformity of the perforation in the tipping paper or the porousity of porous tipping paper should that be used. It is desirable to eliminate this dependence upon the characteristics of the tipping paper.